Unless otherwise indicated herein, approaches described in this section are not prior art to the claims listed below and are not admitted to be prior art by inclusion in this section.
In MU-MIMO wireless communication, multiple antennas simultaneously transmit packets in spatial streams to multiple parties/recipients during a time interval for transmission. The time span of the interval for transmission is usually fixed and determined by the length of the longest packet of the packets being transmitted. One or more of the spatial streams are typically populated in multi-user (MU) groups. However, the payload length of the packets transmitted to some of the recipient groups is sometimes less than the full length available for a given time span.
Conventional approaches supported by the standards (e.g., Long Term Evolution, or LTE, and/or IEEE801.11ac) utilize zero padding at the Media Access Control (MAC) layer. An example scenario 700 is shown in FIG. 7, in which multiple spatial streams are illustrated. In FIG. 7, each spatial stream includes a sequence of time slots (represented by boxes), with spatial stream 1 and spatial stream 3 being transmitted by one antenna and spatial stream 2 being transmitted by two antennas. Specifically, shaded boxes represent time slots with payload data and empty boxes represent time slots with zero padding. In this example, spatial stream 2 has a short payload length. Under the conventional approach of zero padding, padding with bits of 0 and transmission thereof at MAC layer may result in the padded bits being scrambled into a roughly equal number of bits of 0 and bits of 1. Moreover, not transmitting symbols in PHY can mess up channel estimation or create DC offsets. Besides, transmission of padded data typically consumes the same amount of energy as transmission of normal data of a payload even though there is no information content associated with zero padding. Thus, from the perspective of power consumption, padded or scrambled data is indistinguishable from real data.
In transmitting spatial streams, a transmitter usually experiences peaks of increased power as fast Fourier transform (FFT) is uniform while inverse FFT (IFFT) is quite non-uniform. This tends to cause non-linearity in the power amplifier(s) of the transmitter with a resulting peak-to-average power ratio (PAPR) of 8˜10 dB for example. Accordingly, signal transmission and reception may be compromised due to PAPR. Therefore, it is desirable to reduce PAPR to enhance system performance. FIG. 8 shows a 64-tone/sample orthogonal frequency-division multiplexing (OFDM) time domain waveform and power magnitude variations in an example simulation result 800 under a conventional approach. In the example shown in FIG. 8, the magnitude of power varies significantly and this leads to PAPR issue. For large number of tones (e.g., 80 MHz), it is common to have a 10-dB PAPR in the IFFT.